Generally, a fuel cell is an energy conversion device that generates electrical energy through an electrochemical reaction between a fuel and an oxidizer and has an advantage that electric power can be consistently generated as long as the fuel is continuously supplied.
For example, a polymer electrolyte membrane fuel cell (PEMFC), which uses a polymer membrane as an electrolyte capable of permeating hydrogen ions, has a low operating temperature of about 100° C. as compared to other types of fuel cells, and has advantages of high energy conversion efficiency, high output density and fast response characteristics. Besides, since it can be miniaturized, it can be provided as portable, vehicle and household power supplies.
Specifically, the polymer electrolyte membrane fuel cell stack comprises a membrane-electrode assembly (MEA) having an electrode layer formed by applying an anode and a cathode, respectively, around an electrolyte membrane composed of a polymer material, a gas diffusion layer (GDL) for serving to distribute reaction gases evenly and to transfer the generated electric energy, a separating plate (bipolar plate) for supplying reaction gases to the gas diffusion layer and discharging the generated water, and a gasket for preventing leakage of the reaction gases and the cooling water between the membrane-electrode assembly and the separating plate.
FIG. 1 shows a current versus voltage curve of a fuel cell system.
Referring to FIG. 1, a current (I) versus voltage (V) curve of a fuel cell called a polarization curve or performance curve is a representative analysis and expression method for indicating the performance of the fuel cell as data measuring the output current density generated by the fuel cell system under specific operation conditions.
However, such a single characteristic curve shows only limited information on the electrochemical response characteristics and performance changes of the fuel cell to the instantaneous load changes, and an additional diagnostic and analytical technique to be supplemented as a means for determining the dynamic performance change of the fuel cell under the changing operation conditions and the internal state of the operating fuel cell is required.
That is, when the current density changes either from a low state to a high state (forward direction) or from a high state to a low state (backward direction), a hysteresis showing different trajectories appears, but there is a problem that the single current-voltage curve does not include any information on this phenomenon.